John E. Walker

John E. Walker, born in 1941, British chemist and Nobel laureate. Walker shared the 1997 Nobel Prize in chemistry for his discoveries regarding the structure and process of the enzyme, or specialized organic chemical, that converts nutrients into the molecule adenosine triphosphate (ATP), a fuel for virtually every basic life function. Walker’s research confirmed the theories of American chemist Paul D. Boyer and offered insights into the complicated molecular process in which an enzyme called ATPase processes energy into ATP. Walker and Boyer split half of the Nobel Prize. The other half of the prize went to Jens Skou, a Danish chemist who discovered the enzyme that uses ATP while regulating the concentration of ions (positively charged atoms or molecules) in cells.

Walker was born in Halifax, England. In 1964 he earned a bachelor’s degree in chemistry at the University of Oxford in Oxford, England. Four years later he completed his Ph.D. degree at Oxford. Walker spent the next two years as a postdoctoral fellow at the School of Pharmacy of the University of Wisconsin in Madison. From 1971 to 1974 he studied at two research institutes in France. In 1974 he became a member of the scientific staff of the Medical Research Council’s (MRC) Laboratory of Molecular Biology in Cambridge, England. MRC is a publicly and privately supported research center. He became senior scientist for MRC in 1982.

Walker won the Nobel Prize for his contributions to the understanding of the enzyme ATPase and the molecule ATP, which are central to the most fundamental and necessary functions of life. When nutrients from food or sunlight are processed, chemical energy is released in the form of hydrogen ions (H⁺, positively charged hydrogen atoms). The enzyme ATPase uses the hydrogen ions to assemble ATP from the molecules of a chemical called adenosine diphosphate (ADP) and phosphate ions (PO₄^3-), and transfers the ATP into cells. In cells, ATP can fuel any of a number of functions that require energy–from the growth of cells to the contraction of muscles to the transmission of nerve messages. ATP is a fuel for virtually all processes that require energy in a living organism. Figuring out how nutrients are converted into fuel for cells had been a challenge for biochemists since ATP was discovered in 1929.

In the 1980s Paul D. Boyer theorized that the ATPase functions as a three-chamber rotating motor, creating ATP from ADP and phosphate ions, and proposed that the movement was driven by the flow of hydrogen ions. The ATPase enzyme consists of three sections: a six-chambered ball-shaped upper part, a slender rod-shaped middle section, and a cylindrical wheel-like bottom section in the membrane of a cell. The bottom two portions of the enzyme are free to rotate, but the top of the enzyme is held in place by a thin filament that anchors it down to the cell membrane. The top two portions of the enzyme extend into the cell.

Cell respiration, or the process of combining sugars with oxygen, releases hydrogen ions into the liquid between the cells (see Respiration: Internal Respiration). Because the ions all have a positive electrical charge, they seek to move toward an area less crowded with other positive charges. The inside of the cell has a lower concentration of hydrogen ions, so the ions outside the cell try to move to the inside of the cell. The enzyme ATPase offers them a way to get into the cell. Ions enter the cylindrical section and travel through the enzyme into the cell. The ions’ movement through the enzyme causes the bottom two sections of the molecule to rotate. This rotation causes three of the chambers in the top section of the enzyme to twist open and closed. At any one time, one of the chambers is open, one is closed around a molecule of ADP and a phosphate ion, and one is squeezed tightly around an ATP molecule. As the ions flow and the lower parts of the enzyme turn, the chambers take in ADP and phosphate, squeeze them together to form ATP, and open to release the ATP into the cell.

Boyer’s theory was not widely accepted at first. However, in 1994 Walker and his team used X rays to get a better view of the enzyme. Walker’s examination of the structure confirmed Boyer’s theory. Walker further explained the complicated structure and process of the ATPase. His findings helped show how the ball-shaped part of the enzyme produces ATP.